This invention relates to a process for preparing an extract from ground, roasted coffee, which comprises preparing a primary extract by exhaustive primary extraction with water or an aqueous solution in one or more extraction cells including one cell to which extraction liquid is supplied; and subsequently preparing a secondary extract by secondary extraction with water or an aqueous solution in one or more extraction cells including one cell from which secondary extract is withdrawn; the process being conducted so that when the primary extraction cell to which the water or the aqueous solution is supplied has been exhaustively extracted, this cell is connected to the secondary extraction as the extraction cell from which secondary extract is withdrawn.
In the extraction of coffee for the preparation of instant coffee or coffee concentrate, two separate extraction stages are distinguished, namely, the primary extraction and the secondary extraction.
Primary extraction is generally carried out at a temperature maximally equal to the boiling point of water at atmospheric pressure. Under these conditions, only those components which are directly soluble in water are extracted from the coffee. Such an extraction corresponds approximately to the method used in making coffee at home. In such a method, using a very finely-ground coffee, a high water/coffee ratio (&gt;15) and an extraction time of about 10 minutes, a coffee extract is obtained which is very good in quality. From the point of view of quality, this is the optimum manner of preparing coffee extract. However, on an industrial scale, this manner of preparation is not economically justified owing to the low efficiency (&lt;30%).
In order to obtain a sufficiently high extraction efficiency in the extract, in industrial coffee extraction for the preparation of instant coffee or coffee concentrate, a secondary extraction is used as well. The coffee exhaustively extracted in the primary extraction is subjected to such conditions that a portion of the originally insoluble solid in the coffee is hydrolyzed to form soluble components which are then extracted in the secondary extraction. Conventional temperatures for this secondary extraction range between 120.degree. and 190.degree. C. Such extractions are generally carried out in a battery consisting of a plurality of series-connected extraction cells.
When such a coffee extract preparation is compared with the domestic way of making coffee, the following differences are noted:
1. The efficiency of the primary extraction is considerably lower. The reasons for this are:
aa. The use of large particles to minimize pressure drop in the cells.
bb. The use of a low draw-off factor, as a result of which a relatively low amount of hydrophobic aroma compounds are extracted. The presence of sufficient hydrophobic aroma compounds in an extract is of great importance to produce a flavor that is comparable to coffee made in the domestic way.
cc. The primary section is fed with an extraction liquid which already contains extracted material from the secondary section. As a consequence, the extraction in the primary section is much less efficient.
2. In the secondary extraction, an amount of secondary material (extracted solids) is extracted in a proportion approximately equal to the proportion of primary material. This secondary material adversely affects the quality of the extract owing to the presence of off-flavors. However, from the point of view of economy, it is necessary to use the secondary solids.
3. The residence time of the coffee and the extract in the extraction battery is extremely long. In combination with the conditions used, this leads to a deterioration in quality, which is manifested in the taste of the coffee made from the extract.
Part of these drawbacks can be overcome by using so-called split extraction In the split-extraction technique, the primary and the secondary extractions are fed with an extraction liquid independently of each other, so that a primary and a secondary extract are obtained separately from each other. An example of such a process is described in German Offenlegungschrift 27 41 524. In accordance with that process, the coffee is first exhaustively extracted in a primary extraction, subsequently heated in the absence of extraction liquid, and finally extracted further in a secondary extraction. The extracts thus obtained are then concentrated and mixed with each other. In this way, a clearly improved extract quality was obtained relative to split extraction without intermediate heating.
Methods for the split extraction of coffee have also been described in a number of other patent publications. In this connection reference can be made, by way of example, to U.S. Pat. Nos. 3,790,689, 3,810,766, EP-A-90 562, EP-A 97 466 and EP-A-159 754. The processes described in these publications, however, all have the disadvantage that rather a low draw-off factor is used, so that a relatively small amount of hydrophobic aroma components is extracted, with all adverse results of that. An additional consequence is that no exhaustive extraction is obtained in the primary section, so that a part of the primary components is exposed to the adverse effect of high temperature in the secondary section.
Canadian patent 980 165 discloses a process for the extraction of coffee in which the secondary extract is split into a product stream and a feed stream, which feed stream is used as an extraction liquid for the primary extraction.